A Concise Introduction to Practical LTE Systems

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A Concise Introduction to Practical LTE Systems

• Pho Hale

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Physical Perspective

• An eNodeB communicates through a physical channel
  with a specific UE.

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eNodeB

• To establish a connection to an LTE network, a UE
  must first determine the following from the eNB
  base station
• Complete time and frequency synchronization
  information
• Unique Cell Identity
• Cyclic Prefix (CP) Length
• Access Mode (FDD/TDD)
• Synchronization requirements in LTE
• Symbol Timing Acquisition
• Carrier Frequency Synchronization
• Sampling Clock Synchronization

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• Accomplished by broadcasting two special signals
• Primary Synchronization Service (PSS) Used for
  initial synchronization.
• Secondary Synchronization Service (SSS) Used for
  handoff synchronization.
• These synchronization signals are transmitted
  twice per 10ms radio frame.
• For the sake of simplicity, we will assume from
  this point that
• all parameters necessary for communication
  between the UE and the radio-access network are
  known to both entities (RRC_CONNECTED)
• our UE has successfully established a
  synchronized connection to our eNB (IN_SYNC).

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Physical Channel

• Simply the region between the UE and the eNB.
• If the physical channel did not alter the signal
  (i.e. a vacuum) the waveform transmitted would be
  identical to the waveform received.
• If the physical channel altered the signal in a
  predictable and deterministic fashion (i.e.
  attenuation) the waveform transmitted could be
  determined exactly by applying an operation on
  the waveform received.

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• Unfortunately, the physical channel modifies the
  transmitted signal in a random fashion.
• The presence of obstacles and reflectors in the
  communications environment create multiple paths
  a transmitted signal can traverse.
• Each copy of a transmitted signal experiences
  differences in attenuation, delay, and phase
  shift.
• This results in either constructive or
  destructive interference when observed at the
  receiver.
• Known as multipath fading

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• The multiple copies of a transmitted signal
  generated by multipath fading arrive at the
  receiver at different times, inducing and effect
  known as time dispersion.
• In the frequency domain, time dispersion
  corresponds to a non-constant channel frequency
  response.
• Flat Fading
• All frequency components experience the same
  magnitude of fading.
• Frequency-Selective Fading
• Different frequency components experience
  uncorrelated fading.
• In LTE, the downlink (eNB -gt UE) uses OFDM to
  ameliorate performance detriments due to
  frequency-selective fading.

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OFDM

• OFDM divides a wideband signal into tightly
  packed narrowband subcarriers.
• Each subcarrier is now exposed to flat fading,
  rather than frequency-selective fading.
• Two modulated OFDM subcarriers are mutually
  orthogonal over a given time interval, known as a
  symbol length, and denoted T_u.
• Unfortunately the delay introduced by multipath
  fading can shift one subcarrier out of the
  interval of inter-subcarrier orthogonality,
  resulting in interference between subcarriers.

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• To solve this interference problem, the duration
  of each symbol interval is extended by
  duplicating the last portion of a symbol and
  adding it as a prefix to the symbol.
• This is termed cyclic-prefix insertion, and it
  extends the symbol time from T_u to T_u
  T_cp.
• As long as the span of the time dispersion is
  shorter than T_cp, the problem is solved.
• One last issue a frequency-selective channel
  still has the ability to strongly attenuate a
  given subcarrier.

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• Channel Coding spreads out each bit of
  information over multiple code bits.
• These code bits may be distributed in the
  frequency domain over the entire transmission
  bandwidth, a process known as frequency
  interleaving.
• This provides frequency diversity to each data
  bit, ensuring resilience to the failure of any
  specific subcarrier.

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User Equipment

• Concerned with power consumption.
• Assigned one or more Resource Block (RB)

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Logical Perspective

• A Resource Grid represents a 2D data structure
  over both the time and frequency domain.

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Time-Domain Resources

• Variable Symbol Duration
• Time quantum

Duration (ms) Number of Quanta ( )
Radio Frame 10 307200
Subframe 1 30720
Slot 0.5 15360
Symbol 0.5/7 2048
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Frequency Domain Resources

• Subcarrier spacing of 15 kHz
• LTE bandwidth is highly flexible
• 1 MHz to 20 MHz is standard, even more with
  carrier aggregation.

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Algorithmic Perspective

• Both the receiver and the transmitter implement
  methods to reduce the detrimental effect of the
  channel.

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Adaptive Processing

• In dynamic rate control, a transmitter
  dynamically adjusts the data rate in response to
  varying channel conditions.
• The data rate is increased by increasing the
  channel coding rate and/or modulation scheme when
  the channel is clear, and decreasing them under
  disadvantageous conditions.
• Transmission power is kept constant
• Also known as Adaptive Modulation and Coding
  (AMC)
• Channel conditions can be determined by the
  receiver by analyzing a transmitted pilot or
  reference signal.
• These results are then reported to the
  transmitter.

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Error Correction and Retransmission

• Forward Error Correction (FEC)
• Goal Introduce redundancy to a transmitted
  signal to allow bit errors to be corrected by the
  receiver.
• These redundant bits, known as parity bits, are
  computed by applying a coding scheme to the
  information to be transmitted.
• Automatic Repeat reQuest (ARQ)
• Goal Introduce redundancy to a transmitted
  signal to allow bit errors to be detected by the
  receiver.
• The receiver validates all received packets
• if valid receiver notifies transmitter with an
  ACK, data is used
• if invalid receiver notifies transmitter with a
  NAK, data is discarded
• Hybrid ARQ (HARQ)
• Uses FEC to correct a subset of all errors,
  falling back to ARQ for uncorrectable errors.
• May optionally use soft combining
• Packets deemed uncorrectable are stored in a
  buffer for later use instead of being discarded.

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DL-SCH

• CRC Generation
• 24-bit CRC appended to each transport block (TB)
• Segmentation
• TB is split into smaller code blocks, each with
  their own 24-bit CRC.
• Coding
• Turbo Coding is used to encode each code block in
  parallel.
• Rate Matching - HARQ
• each code block is interleaved with a circular
  buffer
• the appropriate number of code bits is selected
  based on cell parameters to be transmitted within
  a given subframe.
• Scrambling
• the block of code bits is XORed with a
  cell-specific scrambling sequence.
• ensures any potentially interfering signals are
  randomized, appearing as gaussian white noise to
  a receiver.
• Modulation
• transforms the block of scrambled bits to a
  corresponding block of complex modulation symbols.

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• References
• Dahlman, Erik, Stefan Parkvall, Johan Skold, and
  Per Beming. 3G Evolution HSPA and LTE for Mobile
  Broadband. Amsterdam Academic, 2008. Print.
• Dahlman, Erik, Stefan Parkvall, and Johan
  Sköld. 4G LTE/LTE-Advanced for Mobile Broadband.
  2nd ed. Amsterdam Elsevier/Academic, 2011.
  Print.     
• Zarrinkoub, Houman. Understanding LTE with
  MATLAB From Mathematical Modeling to Simulation
  and Prototyping. N.p. Wiley, n.d. Print.